[llvm] SystemZ: Handle copies between gr64 and fp64 (PR #124890)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Well, there is not any data type that is legal in both gr64 and fp64, so these copies would never come up. I'm wondering why this is needed now?

There would still be bitcasts, which I would expect to emit a copy. Peephole optimizer doesn't do a great job looking through various subregister patterns, which I'm working on fixing. Uncoalescable copies and bitcasts get rewritten 

https://github.com/llvm/llvm-project/pull/124890